1. Field of the Invention
The present invention relates generally to stretchers and compressors for lasers. In particular, the present invention relates to a dispersive laser pulse stretcher and compressor with single parameter wavelength tunability.
2. Description of the Related Art
Chirped Pulse Amplification (CPA) provides a technique for amplifying short laser pulses to previously unattainable intensities. This technique involves dispersively stretching a short laser pulse prior to amplification, thus decreasing the peak power of the laser pulse during amplification. After amplification, the stretched pulse is compressed in a dispersive compressor which provides extremely high peak intensities.
The CPA systems of the prior art employ a variety of stretchers and compressors. FIG. 1 illustrates a two grating pulse stretcher 10 of the prior art. As shown in FIG. 1, the beam enters the stretcher 10 and is reflected by a first mirror 12 to a first diffraction grating 18. The grating 18 passes the diffracted beam on through a first and a second lenses 22, 24 to the second diffraction grating 20. The diffracted beam is incident on grating 20 at the same angle (relative to the grating normal) that it left the first grating 18. The second grating 20 directs the beam to a first vertical retroreflector 14. The first vertical retroreflector 14 reflects the beam on a return path to the second grating 20, the second lens 24, the first lens 22, the first grating 18, and the first mirror 12. The beam is then output by reflecting the beam from the first mirror 12 at a different height than the input beam, allowing mirror 16 to reflect the beam out of the stretcher.
FIG. 2 illustrates another prior art pulse stretcher 26. The pulse stretcher 26 of FIG. 2 only employs a single grating. As shown, the beam enters the stretcher 10 and is reflected by a first mirror 12 to a first grating 18. The first grating 18 passes the diffracted beam on through a first lens 22 to a first vertical retroreflector 14. The first vertical retroreflector 14 reflects the beam back through the first lens 22 to the first grating 18. The symmetry provided by the vertical retroreflector 14 makes this optically identical to the stretcher 10 of FIG. 1. The return path of the beam is completed by traveling from the first grating 18 through the second vertical retroreflector block 28 to the output by way of the first and third mirrors 12, 16.
Similarly, single and dual grating pulse compressors have also been developed. FIG. 3 illustrates a two grating pulse compressor 30. The compressor 30 comprises a first and third mirrors 12, 16; a first vertical retroreflector 14; and a first and second gratings 18, 20. The stretched beam enters the compressor 30 and is reflected by the first mirror 12 to the first grating 18, then to the second grating 20 until the first vertical retroreflector 14 is reached. The beam approaches the second grating 20 at the same angle (relative to the grating normal) that it left from the first grating 18. The first vertical retroreflector 14 reflects the beam back to the second grating 20 and to the first grating 18. The returning beam is then reflected by the first mirror 12 and the third mirror 16 to the output.
A single grating pulse compressor 32 of the prior art is shown in FIG. 4. The single grating pulse compressor 32 comprises a first, second, third, and fourth mirrors 12, 15, 16 and 34; a first vertical retroreflector 14; and a first grating 18. The beam enters the compressor 32 and is reflected by the first mirror 12 to the grating 18. The beam leaves the grating 18 and returns to the grating 18 at the same angle it left with, after being reflected by the second and third mirrors 15, 16 at right angles to each other. The beam is then directed to the first vertical retroreflector 14. The beam returns from the first vertical retroreflector 14 and follows a path through the grating 18, to the third and second mirrors 16, 15, to the grating 18 and then to the first mirror 12. The compressed beam is then output after being reflected by the fourth mirror 34.
One continuing problem with the CPA systems of the prior art has been aligning the input of the stretcher and the input of the compressor. Various combinations of the stretchers and compressors described above can be used, but in order for stretching and compressing to work properly, the input angle of the beam into the stretcher must be adjusted to be precisely the same as the input angle of the beam into the compressor. The input angle can be adjusted by rotating the position of the gratings about their central axes. In systems using multiple gratings, the alignment of the input angles to the stretcher and compressor is very time consuming and laborious since the input angle for each grating must be modified to assure the proper alignment. For example, a typical alignment process for just the stretcher includes repeating the steps of: adjusting the position of the first grating for the stretcher, measuring the input angle to the first grating, modifying the input angle for the second grating, and measuring the input angle for the second grating. The process for measuring the input angle is very laborious and time consuming. It is not uncommon for these steps to be repeated numerous times to obtain the proper input angle just for the stretcher, and the alignment process often takes several hours. Once the stretcher has been aligned, then the compressor must be precisely aligned to have the same input angle. Aligning the compressor is performed in similar manner by repeating the steps of repositioning the gratings and then measuring the input angles. Similarly, the alignment process for the compressor can take several hours.
The significance of the problems involved in properly aligning the stretcher and compressor are heightened by the fact that the stretcher and compressor must be realigned every time a different wavelength pulse is stretched and compressed. Unfortunately, the grating angles for proper operation of CPA are different for different wavelengths. Therefore, each time the laser wavelength is changed, the alignment procedure must be performed for the stretcher and compressor to operate properly.
One prior art method for avoiding the problems associated with realigning the gratings is to make a stretcher and compressor with optical components, namely diffraction gratings, large enough to allow some degree of tunability without having to move them. This is presently not practical due to the technological limitations on the size of the diffraction gratings commercially available. Even using the largest diffraction gratings that are currently available, the grating size would continue to limit tunability. Moreover, even if larger diffraction gratings were able to provide the tunability desired, their cost would make any system including them prohibitively expensive.
Therefore, there is a need for a pulse stretcher and compressor that is very easy to adjust for correct operation of the CPA, or for changes in the wavelength of the laser begin amplified.